ConceptArt by Inkthinker.
At 27 he was seen as betraying his people in a fundamental way, for which he was named Truthless, cast out and sold to a trader named Vstim, who acquired him as a servant. His betrayal is suspected to relate to claims that Voidbringers and the Knights Radiant would return.
Assassin in WhiteEdit
His first bout as the assassin in white is when he is hired by the Parshendi to assassinate King Gavilar, though assassinate may not be the exact word. He puzzles over why the Parshendi would break their treaty, but decides it is not his place to ask. It is here that the first mentions of Surgebinding are shown.
He leaves and is picked up by Avado, with some half-truths and lies, he convinces Avado to bring him away. Szeth is passed from one master to another by means of an Oathstone, a small dark rock flecked with quartz. Szeth believes he eventually makes his masters uncomfortable; he thinks it is because most of them know he is capable of more than he does for them, and that they are afraid that somone else will find out that they control him simply by using the Oathstone.  Szeth is 35 years old, and has been Truthless for 7 years at the time he is owned by Took.
He is later acquired by Makkek used to expand his power, and to scare his competitors. On a mission to slay a competitor, he is beaten to his target and is approached to be used to his 'full potential' by a mysterious man, who has killed Makkek and taken the oathstone.
This new master realizes Szeth's potential and creates a list of targets for Szeth to kill. Several kings and other government leaders are on the list.
After having killed all but the last on the list, Taravangian, he prepares to kill Taravangian only to realize that it was Taravangian who ordered the deaths.
According to Kharbranth king Taravangian, Szeth's status as Truthless includes an absolution for all future sins committed while following his masters' orders, but Szeth dismisses this as a misconception and takes responsibility for all the murders he commits, but it doesn't seem to impede anything but his sanity.
Words of Radiance Edit
Szeth is commanded to assassinate Highprince Dalinar by his current master, Taravangian. During his first attempt, he fights with Adolin, Kaladin, and Dalinar, who all believe Szeth intends to kill Elhokar. Szeth has no trouble defeating Adolin, and manages to cut Kaladin's forearm with his Honorblade. Kaladin ultimately tackles Szeth, sending them both falling through a hole in the wall. Szeth realizes that Kaladin is a Surgebinder when Kaladin uses Stromlight to restore function to his lifeless hand. Szeth then flees, as he realizes that if the Surgebinders have returned, he is not Truthless, and his slavery would have been based on a lie.
He returns to Taravangian with his concerns, and Taravangian lies to Szeth, claiming that Kaladin must have stolen an Honorblade from Shinovar. This is enough to persuade Szeth to return to the Shattered Plains to complete his assignment of assassinating Dalinar.
Szeth arrives at the Shattered Plains during a battle between the Alethi forces and the stormform Parshendi, just before the Parshendi summon the Everstorm. He is discovered by Adolin Kholin as he is making his way to Dalinar. He defeats Adolin easily, and engages Dalinar in single combat. Szeth beats Dalinar, but is then attacked on all sides by Alethi forces, trying to drive him back. Szeth easily dispatches many of them, killing Brightlord Roion by Lashing him into the sky. He then Lashes Dalinar upwards as well, but Kaladin arrives in time to save him with a Lashing of his own.
Kaladin and Szeth fight in the air, using Surgebinding to fly. During his fight with Kaladin, Szeth accepts that the Knights Radiant have returned, meaning he is not Truthless and is free to choose his own path, and allows Kaladin to kill him. (Ch. 86, p. 1041, Kindle location 20859). Kaladin then takes Szeth's Honorblade.
He was revived By the Herald Nalan (Ch. 88, p. 1062, Kindle location 21776). He was then given a new Shardblade. His new Shardblade is described as being black, it seems to leave a "small trail of black smoke. Like Stormlight, but dark." 
This could be a reference to another of Brandon Sanderson's books, Warbreaker. The blade may be an awakened sword from the Warbreaker series, potentially Nightblood itself. An awakened sword is created by using thousands of Breaths to bring an inanimate object to life and giving it a command to follow. The command given to Nightblood is to "Destroy Evil." The only problem with this is that an inanimate object doesn't have a great understanding of what evil truly is. The sword given to Szeth does match the description of Nightblood, has the sentient voice that occurs in the wielder's mind, and emits black smoke like Nightblood does. In addition, the first words Szeth heard in his head after seeing the sword were "Hello. Would you like to destroy some evil today?"
Former Masters of Szeth-son-son-VallanoEdit
Part of Szeth punishment is for him to do whatever the holder of his Oathstone, his master, commands. Throughout the The Way of Kings, most everything Szeth does is at the behest of others. Among his masters are:
- Vstim the trader who first bought him.
- A Cobbler.
- A traveling Merchant.
- A farmer in a village near the mining village of Ironsway in Bavland.
- Makkek - Killed Took and took possession of Szeth. Makkek became the owner of a gambling den in Bornwater, Bavaland, using Szeth as his body guard and assassin.
- Liss - An assassin, who sold Szeth to the Parshendi just prior to the assassination of Gavilar
- Parshendi during to the assassination of Gavilar
At the beginning of the story, he is aggravated by the fact that stone (considered holy in Shin culture) is used as a common building material. However, as the story progresses, he becomes more accustomed to it, though never approving of it.
He also seems to be partially insane as he can hear the screams of the dying from behind furniture, in cupboards, etc...
He has a sense of morality, shown in the fact that he almost kills Taravangian to stop the deaths of people within the secret room of the Conclave. Besides that, he also hates to kill and weeps as he kills.
Near the end of his slavery, he begins to blame his victims for not being able to stand up to him, wishing that someone would end his accursed existence.